1. Field of the Invention
This invention relates to a system for controlling the output power of the laser beam which oscillates in accordance with pulse discharge and, more particularly, to a device for maintaining a constant total output power of a laser.
2. Description of the Prior Art
Various methods have been proposed for obtaining a stable laser output from ordinary discharge-exciting type laser, such as negative feedback of the load current to the power supply, thereby controlling the discharge current. A pulsed laser, such as excimer laser, is difficult to obtain a constant laser output power in each pulse discharge.
On the other hand, a pulsed laser can suitably be used in carrying out photolithography in a semiconductor process. In a photolithographic process, even though equal number of pulses are input to successive steps and repeated cycles, the amount of exposure varies each cycle, due to a fluctuation in the reproducibility according to the pulse cycles. It has been difficult so far to obtain a process which only has small fluctuation.